Biaxially oriented polypropylene slip films used for packaging applications often perform multiple functions. They must perform in laminations as slip films with low and stable COF; and they must perform as print films with good optical clarity, gloss, and ink adhesion.
Traditional methods for producing a functional slip film have often relied upon relatively high amide-type additives. These fatty amides (e.g. erucamide, stearamide, behenamide), typically in concentrations of 2000-5000 ppm, bloom to the surface of the film. The amide molecule form bumps on the polymer surface, thus reducing surface-to-surface contact and lowering the COF. However, the drawback of such additives is their migratory behavior in polyolefin films. The amount of additive bloom to the surface can vary with environmental conditions, and thus slip properties can vary greatly depending on storage conditions seasonal changes in humidity and temperature, or geographical differences in temperature and humidity. Warmer storage conditions tend to enhance blooming of these amides, whereas cool storage conditions tend to slow the migration process. If too little additive blooms, the laminated film may exhibit high COF and cause web breaks on the packaging machine's forming collar or sealing bars from excessive friction. If too much additive blooms, the laminated film may exhibit a COF that is too low for the packaging machine's tension control system, causing wrinkling or “flooding” of the line. Because of the variability in storage conditions, the same slip film using these amide systems can exhibit both extremes of COF properties.
In addition, the use of fatty amide additives also tends to make the film hazy and less glossy. This is undesirable from the point of view of print graphics appeal. Moreover, the migratory nature of the amide additive packages make them prone to bloom to the print surface of the slip film or transfer from the slip surface to the print surface when in wound roll form. The presence of amides on the print surface can interfere with the wettability and adhesion of water-based inks and, especially for process print applications, cause bridging of ink dots or inconsistent dot sizes. This results in muddier colors or images and a loss of graphic appeal.